A successor communication system to wideband code division multiple access (W-CDMA) and high speed downlink packet access (HSDPA), i.e., Long Term Evolution (LTE), is currently being discussed by 3GPP, a standardization group. In LTE, orthogonal frequency division multiple access (OFDMA) is to be used as a downlink radio access method and single-carrier frequency division multiple access (SC-FDMA) is to be used as an uplink radio access method (see, for example, 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006; and 3GPP TS 36.300 (V8.2.0), “E-UTRA and E-UTRAN Overall description,” September 2007).
In OFDMA, a frequency band is divided into multiple narrow frequency bands (subcarriers) and data are transmitted on the subcarriers. The subcarriers are densely arranged along the frequency axis such that they partly overlap each other but do not interfere with each other. This method enables high-speed transmission and improves frequency efficiency.
In SC-FDMA, a frequency band is divided into multiple frequency bands and the frequency bands are allocated to different terminals for transmission in order to reduce interference between the terminals. Also, SC-FDMA reduces variation of the transmission power and therefore makes it possible to reduce power consumption of terminals and to achieve wide coverage.
In an LTE communication system, shared channels are used for downlink and uplink.
In downlink, for example, a base station selects, for each subframe (e.g., 1 ms), user equipments allowed to communicate using the shared channel and transmits the shared channel to the selected user equipments. The base station also transmits a downlink control channel in addition to the shared channel to report to the user equipments that the shared channel has been transmitted in the subframe.
In uplink, for example, the base station selects, for each subframe (e.g., 1 ms), user equipments allowed to communicate using the shared channel and transmits a downlink control channel to the selected user equipments to request them to communicate using the shared channel in a specified subframe. The selected user equipments in turn transmit the shared channel based on the downlink control channel. Then, the base station receives and decodes the shared channel transmitted from the user equipments.
A process of selecting user equipments allowed to communicate using the shared channel is called “scheduling”.
Popular algorithms for the scheduling include a round robin algorithm and a proportional fairness algorithm.
In the round robin algorithm, radio resources for the shared channel are allocated in rotation to user equipments in a cell.
The proportional fairness algorithm is described below. In the proportional fairness algorithm, for example, a coefficient Cn (n indicates an index of a user equipment) is calculated for each user equipment using formula 1 below, and radio resources for the shared channel are allocated to one or more user equipments whose coefficients Cn are the largest.
                                          C            n                    =                                    Q              n                                                      R                _                            n                                      ⁢                                  ⁢                              Q            n                    ⁢                      :                    ⁢                                          ⁢          Instantaneous          ⁢                                          ⁢          channel          ⁢                                          ⁢          quality                ⁢                                  ⁢                                            R              _                        n                    ⁢                      :                    ⁢                                          ⁢          Average          ⁢                                          ⁢          transmission          ⁢                                          ⁢          rate                                    [                  Formula          ⁢                                          ⁢          1                ]            
For downlink, Qn is calculated, for example, based on a downlink channel quality indicator (CQI) reported by the user equipment. For uplink, Qn is calculated, for example, based on the radio quality such as the signal-to-interference ratio (SIR) of a reference signal transmitted from the user equipment.
Generally, in a mobile communication system, there are data with a high priority and data with a low priority. For example, a dedicated control channel (DCCH), i.e., C-plane data, has a high priority and a dedicated traffic channel (DTCH) transmitted based on a best-effort service has a low priority. Data of a user subscribing to a high-priced plan have a high priority and data of a user subscribing to a low-priced plan have a low priority. Also, delay-sensitive data such as VoIP data and streaming data have a high priority and packet data transmitted based on a best-effort service have a low priority. To perform scheduling taking into account the priorities as described above, the coefficient Cn may be defined as follows:
                                          C            n                    =                                    A              n                        ·                                          Q                n                                                              R                  _                                n                                                    ⁢                                  ⁢                              A            n                    ⁢                      :                    ⁢                                          ⁢          Priority          ⁢                                          ⁢          level                                    [                  Formula          ⁢                                          ⁢          2                ]            
Introducing the priority level An into the formula for obtaining the coefficient Cn makes it possible to perform scheduling taking into account the priority of data.